For example, in a pressurized light-water reactor or an integrated modular light-water reactor, primary coolant in a pressurized state is circulated between a reactor core and a steam generator, and the heat of the primary coolant is transmitted to secondary coolant by the steam generator, to thereby generate steam. Here, inside the steam generator there are arranged a plurality of pipe lines bent in a U-shape and through which passes the primary coolant which circulates between the reactor core and the steam generator. By filling the surroundings of these pipe lines with the secondary coolant, the heat is transmitted from the primary coolant to the secondary coolant via the pipe lines, and this secondary coolant is evaporated, thereby generating steam. At this time, the U-shape pipe lines arranged inside the steam generator receive vibration from the primary coolant passing inside, and vibrate upon receiving an exciting force from the secondary coolant which is boiling and flowing as a two-phase flow.
Consequently, in the steam generator described above, a two-phase flow is flowed around the pipe lines on a trial basis to measure the void fraction of the two-phase flow, and the vibration characteristic of the vibrated pipe line is measured. Then, the exciting force which acts from the flowing two-phase flow on the pipe lines is associated with the void fraction and evaluated, and the pipe lines and members which fix these pipe lines are designed based on the results of this evaluation. Here, the void fraction of a two-phase flow can be found by arranging electrodes respectively on the inner surface and in the center portion of a flow path along which the two-phase flow travels, and measuring voltages occurring between these electrodes (for example, refer to Patent Document 1). Moreover, the vibration characteristic of the pipe line can be obtained by measuring a displacement occurring as a result of vibrations of the pipe line, using a displacement sensor, or by measuring stress occurring as a result of vibrations of the pipe line, using a stress sensor.
However, in the above evaluation of exciting force of two-phase flow, it is necessary to provide, independently in different positions, equipment that measures a two-phase flow void fraction on a flow path along which the two-phase flow travels, and equipment for vibrating the pipe line and measuring the vibrations thereof. Consequently, there is a problem in that the entire device including the flow path becomes large, and installation thereof is troublesome. Moreover, since each piece of equipment needs to be installed in different positions, there is a problem in that the measurement result of the measured two-phase flow void fraction and the measurement result of the vibration of the corresponding pipe line need to be synchronized, and it is difficult to make accurate measurements and measurement is time consuming.